I've made this device to connect my NTC thermistor to a Livolo thermostat which only works with a built-in DS18B20 digital thermometer.

Instructions

table.h

You need to generate the table.h for your particular NTC thermistor. It contains an array which maps a measured ADC value to actual temperature. You can do this via the provided table.py script.

Measure your NTC resistance at two different temperature points. Then call table.py specifying your measurements:

python table.py \
    --t1 <temperature-1> --r1 <resistance-at-t1> \
    --t2 <temperature-2> --r2 <resistance-at-t2> \
    --rf <fixed-resistor>

If your program memory is too small, you can specify --min-t and/or --max-t parameters. Then the table will be generated in a specified temperature range generating less array entries thus taking less memory.

See Measuring the temperature with NTCs for more information on how the conversion is implemented.

Schematic

• R2 is a fixed resistor with a known resistance. Start with a one that is close to your typical NTC resistance. I recommend to measure actual resistance of the resistor before passing it as --rf to improve readings accuracy.
• C1 aims to filter noise on the NTC wires. It may be tempting to remove it but then you'll sometimes get inadequate readings. You may adjust it though depending on how fast temperature changes in your setup.
• I use MISO pin for 1-Wire data line because then I'm able to solder the power and data wires close to each other on the ICSP header.
• I used a self-fusing electrical tape to wrap the entire Arduino board and put it into a flush-mounted box behind the thermostat.

Note for Livolo thermostats

In short, damn them! 😡

1. Livolo's 1-Wire master has non-standard timings and reads only the first 2 bytes of the entire DS18B20's response. You'll need to patch OneWireHub's OneWireHub_config.h and DS18B20.cpp using the included livolo.patch. See also orgua/OneWireHub#84.
2. (Still checking this point.) Supposingly, one has to solder out the second sensor deep inside the thermostat even before turning it on for the first time. If you've already done that, you may try to figure out the calibrated temperature offset that the thermostat has calculated and remembered. Wait ~4 minutes and note the difference between displayed and real ambient temperatures. Then, change livoloOffset in the source code to compensate this.